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The EJ253 in my 2005 (MY06) SG manual Forester is due for some maintenance. For negligible extra cost I can get a 2010 EJ253 out of a SH automatic Forester with 190,000Km less. My current engine isn't bad, so I'll only consider the swap if it's straight forward.

So far I believe that the SH motor will fit straight in and the manual transmission will bolt straight up.

I'm unsure about the differences in the electronics between manual and auto motors. Will my manual ECU and all of the sensors plug straight into the auto motor and run it with out issue or are there some fundamental differences between the two motors?

At the same cost (approximately), I would do the required maintenance on your existing engine.

Short of doing the same maintenance on the SH engine (extra cost), you cannot be assured that you are exchanging one set of problems for another.

Doing this also avoids nasty surprises ... Changing trannies is not as straightforward as one might think, judging from my one experience some 40+ years ago. And that was in the days when an ECU wasn't even on the horizon.

SWMBO's SH has a slightly different engine from that in Roo2, even though both are EJ253 engines.

I would still put on new timing belt, water pump, head gaskets and seals onto the new engine if I went that route. I am keeping my manual gearbox, just wasn't sure how an engine that had been paired with an auto would match.

Compression is just a bit low and uneven on the old engine. I was going to change valve seals and inspect cylinders and valves when the heads are off, any machine work can quickly offset the cost off swapping engines. While I don't know the history on the other motor, I do know mine has not had an easy life, however does still run really nice.

I know they can turn into a headache. My mate upgraded his Ford with a Tickford motor. Took him a very long time to get it running and changed basically everything on it. For example he wanted an auto but ended up with a manual. End result is an awesome car, but what he went through is exactly what I want to avoid.

Default plan will be to keep my engine. But would be nice if someone has done something similar knows what to look for. Then I can knock a lot of kilometers off.

auto/manual donks are unlikely to be different, but what you need to watch out for are differences in the sensors and engine wiring harness. The ECU's will be different (Auto/Manual same age, let alone different ages. Simple things like neutral sensor, clutch switch, idle speed control etc are all different manual to auto).

Just about all subaru EJ engine/trans combos bolt up with no mechanical hassles.

You want to ensure that the new engine plugs straight in. Same plugs on the engine harness, same sensors, same CR, etc. If injectors differ, they need to be swapped to your old ones.

If the sensors/wiring differ, then the next step is to use the new long block and your current heads/intake etc. Again, you need to be sure CR is ok. But if your old heads need work, this seems a bit silly.